The primary method of manufacturing phosphoric acid is by digestion of phosphate rock with acid. This is commonly referred to as the "wet" process for phosphoric acid production. Apatite (calcium phosphate) is the most commercially valuable phosphate mineral. Sulfuric acid is the most commonly used acid for digestion. A by-product of wet process phosphoric acid production is the generation of a significant amount of calcium and magnesium sulfates (commonly referred to as phosgyp), and to a lesser degree, other precipitates. The insoluble solids are typically removed by filtration, usually on a horizontal pan filter. The solids are then discharged as a waste product and the filtered acid is concentrated by evaporators with a clarification process between each stage. The final acid concentration is determined by the end use and can be as high as 67 to 76% P.sub.2 O.sub.5, which is known as superphosphoric acid. Fertilizer production accounts for about 90% of phosphate rock utilization.
After the phosphate matrix has been mined, it is beneficiated by washing, screening and/or flotation processes to concentrate the phosphatic material. The concentrated phosphate rock consists of phosphates of calcium, magnesium and the like with some organic as well as inorganic impurities. This material is dissolved by hot (140.degree.-190.degree. C.) sulfuric acid to yield a phosphoric acid solution of about 28% P.sub.2 O.sub.5. The sulfuric acid reacts with the cations in the phosphatic ore matrix, producing insoluble precipitates. The bulk of the insoluble solids consist of sulfates of calcium and magnesium which are removed by filtration prior to concentration of the acid. Additional solids are generated during concentration of the acid and are removed through sedimentation in clarifiers.
Flocculants are often used to aid the clarification process. See, for example, U.S. Pat. No. 3,644,091, which discloses the use of water soluble sulfonated polystyrenes having molecular weights of from about 1 to about 40 million as phosphoric acid clarification aids.
U.S. Pat. No. 4,800,071 discloses the use of sulfonated acrylamide and sulfonated acrylamide/acrylate polymers to aid in the filtration of gypsum in the "wet" process for production of phosphoric acid.
U.S. Pat. No. 4,291,005 discloses the use of acrylamide/acrylate polymers for settling suspended solids in phosphoric acid product solutions. The polymers disclosed consist essentially of a predominant proportion of acrylic acid units and a minor portion of acrylamide units.
Japanese patent No. JP 56/88814 (CA96(2):8835d) discloses the use of acrylamide/(methacryloyloxy) ethyldimethylbenzelammonium chloride copolymers to flocculate solids from phosphoric acid.
Czechoslovakian Patent No. CS204404 B (CA99(26):214925y) discloses the use of melamine formaldehyde and PhNH.sub.2 formaldehyde condensates for clarification of suspended solids in phosphoric acid.
CA86(18):123780 and CA99(12):903x relate to the effect of polyacrylamide on the filtration rate of phosphogypsum.
CA91(24) 195236r discloses the effect of various polyacrylamides and polyethylene oxide polymers on the filterability of phosphogypsum.
By contrast, the present invention relates to the use of sulfonic-type polymers which possess a high anionic charge content as flocculation aids to improve the separation of suspended solids from all grades of phosphoric acid. The term "high anionic charge" refers to the charge density of the polymer in distilled water. It is recognized that a significant portion of the polymer may be protonated at pH levels found in this application. While it is believed that low charge AMPSA/AM-type polymers have been used as phosphoric acid clarification aids, the use of the instant high charge polymers is not disclosed or suggested in the art.